Handoff and source congestion avoidance spread-spectrum system and method

ABSTRACT

A spread-spectrum, code-division-multiple-access (CDMA), system with a remote station (RS) communicating with a first base station (BS). The remote station receives the first BS-packet signal, and transmits a first RS-packet signal to the first base station. The first RS-packet signal is spread by a first RS-chip-sequence signal at a second frequency. The first base station receives the first RS-packet signal. The first base station stores and forwards the despread first RS-packet signal to a central office (CO). The remote station monitors control and packet transmission channels of other base stations in geographic proximity to the remote station. Each of the base stations transmit BS-packet signals. The remote station determines, based on signal metrics and available capacity, when to change from the first base station to the second base station. The second base station stores and then forwards the despread second RS-packet signal to the central office. The despread second RS-packet signal includes a source address from the second base station and the source address from the remote station. For return communications from the central office, the central office reads the source address from the second base station, and routes return packet signals, denoted herein as CO-packet through the second base station to the remote station.

RELATED PATENTS

[0001] This patent stems from a continuation application of U.S. patentapplication Ser. No. 09/758,981, and filing date of Jan. 12, 2001,entitled SPREAD-SPECTRUM HANDOFF AND SOURCE CONGESTION AVOIDANCE SYSTEMAND METHOD by inventors, DONALD L. SCHILLING and JOSEPH GARODNICK. Thebenefit of the earlier filing date of the parent patent application isclaimed for common subject matter pursuant to 35 U.S.C. §120.

BACKGROUND OF THE INVENTION

[0002] This invention relates to packetized,code-division-multiple-access communications, and more particularly tohandoff of a remote station, between base stations.

DESCRIPTION OF THE RELEVANT ART

[0003] In a wireless, direct-sequence, spread-spectrum, packetized,code-division-multiple-access (CDMA) communications system, a remotestation (RS) transmits to a base station (BS), and a base stationtransmits to a remote station. The selection of base station may hedetermined from the power level received at the base station from theremote station, and/or from the power level received at the remotestation from the base station. During communications between the remotestation and the base station, if either the power level received at theremote station, or the power level received at the base station, is toosmall, then unacceptable communications results, and a change of basestation is required or the communications channel is “dropped”. Thechange of base station is referred in the art as a handover or handoff.

[0004] In the prior art, a base station may initiate a handoff, or theremote station may initiate a handoff. The handoff may be a hardhandoff, when communications with the remote station is stopped for ashort period of time, until handoff to a new base station is complete.The hard handoff may result in loss of data. To avoid the loss of data,the data that might be lost can be stored, and when the handoff iscomplete, the stored data can be transmitted at an increased data rateand an increased power level. This sometimes is referred to as store andforward. Ths store and forward of data during the hard handoff canprevent the loss of data.

[0005] For a soft handoff, the remote station, at the same time,receives data from, and transmits data to, the old base station and thenew base station. The simultaneous transmission results in an increasein received signal power, by as much as 3 dB, since two base stationsare transmitting to the remote station.

[0006] For the hard handoff and the soft handoff, initially the remotestation and/or base station determine that a handoff is required, andthen as to which of several base stations the remote station finallywill communicate. For the handoff, an initial remote station transmitterpower is determined, then the necessary overhead operations take placeto effect the handoff. During this process, the hard handoff results indata loss, which is unacceptable for data communications, and the softhandoff results in a decrease in capacity, since two base stations aresimultaneously transmitting and/or receiving the same data. In addition,the handoff procedures currently in use were for circuit-switchedsystems.

SUMMARY OF THE INVENTION

[0007] A general object of the invention is a handoff, for adirect-sequence, spread-spectrum, CDMA packet-switched system, betweenbase stations, without loss of capacity or loss of data.

[0008] Another object of the invention is to provide continuousoperation of a remote station between base stations, without the largeoverhead operation currently required of a handoff.

[0009] According to the present invention, as embodied and broadlydescribed herein, an improvement for a method and system is provided toa spread-spectrum, code-division-multiple-access (CDMA), system. Assumethat a remote station (RS) is communicating with a first base station(BS) within a spread-spectrum, CDMA network. The spread-spectrum, CDMAnetwork may be a star network with the same overhead information whichis typically used in current cellular systems, or a distributed network.The first base station transmits, using radio waves, a first BS-packetsignal to the remote station. The first BS-packet signal is spread by afirst BS-chip-sequence signal at a first frequency f₁.

[0010] The remote station receives the first BS-packet signal. A replicaof the first BS-chip-sequence signal, or equivalently a matched filtermatched to the first BS-chip-sequence signal, is used by the remotestation for despreading first BS-packet signals arriving from the firstbase station. Each BS-packet signal, or certain BS-packet signals,contain capacity availability data, and which BS-chip-sequence signalsare available, for the respective base station.

[0011] The remote station transmits, using radio waves, a firstRS-packet signal to the first base station. The first RS-packet signalis spread by a first RS-chip-sequence signal at a second frequency f₂. Areplica of the first RS-chip-sequence signal, or equivalently achip-sequence generator matched to the first RS-chip-sequence signal, isused by the remote station for spreading the first RS-packet signal. Thefirst RS-packet signal includes a source address from the remotestation.

[0012] The first base station receives the first RS-packet signal at thesecond frequency f₂. The first base station normally would despread thefirst RS-packet signal, and then send the despread first RS-packetsignal to a central office. The despread first RS-packet signal includesa source address from the remote station, and the source address fromthe first base station.

[0013] While the remote station communicates with the first basestation, the remote station also monitors spread-spectrum signals fromother base stations, which are in geographic proximity to the remotestation. The remote station may continuously monitor, periodicallymonitor, or monitor when the received power level at the remote stationfalls below a threshold or when the capacity availability in the firstbase station falls below a given threshold. The remote station maymonitor the control and packet transmission channels, or other channels,of the other base stations. The control and packet transmission channelsrefer to channels that are continuously operating, or nearlycontinuously operating, from the other base stations.

[0014] Each of the base stations transmits, using radio waves, aBS-packet signal spread by a second BS-chip-sequence signal at the firstfrequency f₁. The remote station monitors a signal metric of the controland packet transmission channels from the first base station, andmonitors the same signal metric of the control and packet transmissionchannels from the other base stations. The remote station determineswhen the signal metric of the first BS-packet signal falls below athreshold, and that the signal metric of a BS-packet signal from onebase station, or several BS-packet signals from several base stations,is above the threshold. The remote station also determines availablecapacity of the one or more base stations. Upon determining theforegoing threshold crossings, and capacity levels, the remote stationdetermines to handoff to a second base station. The second base stationis defined herein to be the particular base station, out of the one ormore base stations monitored by the remote station, to which the remotestation decides to handoff. The second base station, as set byengineering design criteria, has sufficiently high signal level andsufficient capacity, for communicating with the remote station. Uponmeeting these criteria, the remote station changes communications fromthe first base station to the second base station.

[0015] A packet switched system makes use of the fact that a packetcontains a finite number of bits and that the remote station transmitsnothing between packets. Even in voice communications, the dead time,i.e., the time between spoken words, including pauses, is about sixtypercent. When a remote station transmits a packet there is a dead time,the time between the remote station transmission of the next packet.This dead time is unknown a priori at the base station and often unknownby the remote station.

[0016] Thus, after transmission of a packet i, the base station will usethe chip-sequence signal and the available capacity in the base station,given to a first remote station, for the next remote station requiringits use. Such operation is required to ensure, maximum throughputthrough the system. Thus, the original remote station desiring to sendpacket i+1, may find that the base station has no capacity available.This metric, denoted herein a BS metric, is a second reason (the firstbeing low power) for the remote station to handover to a second basestation.

[0017] In response to determining to change communications between basestations, the remote station transmits, using radio waves, a secondRS-packet signal, which is called the second RS-packet signal, to thesecond base station, using a newly selected spreading RS-chip-sequencesignal. The newly selected spreading RS-chip-sequence signal, definedherein as the second RS-chip-sequence signal, is provided by the secondbase station. The term second RS-packet signal, as used herein, refersto the initial RS-packet signal transmitted from the remote station tothe second base station, during handoff, using the secondRS-chip-sequence signal. Accordingly, the second base station despreadsthe second RS-packet signal using the appropriate RS-chip-sequencesignal or matched filter technology.

[0018] After the second base station receives and despreads the secondRS-packet signal at the second frequency, the second base station sendsthe despread second RS-packet signal to the central office (CO). Thedespread second RS-packet signal includes a source address from thesecond base station and the source address from the remote station.

[0019] For return communications from the central office to the remotestation, the central office reads the source addresses from the remotestation and the forwarding base station. For this example, the centraloffice reads the source address from the second base station. From thesource addresses, the central office knows to route return packetsignals, denoted herein as the CO-packet signal, through the second basestation to the remote station.

[0020] The central office sends to the spread-spectrum, CDMA network, aCO-packet signal, which includes destination addresses for the secondbase station and the remote station. The second base station receivesthe CO-packet signal. The second base station transmits, using radiowaves, the CO-packet signal, at the first frequency, as a thirdBS-packet signal spread by the second BS-chip-sequence signal. The termthird BS-packet signal, as used herein, refers to the BS-packet signaltransmitted from the second base station, containing the informationfrom the CO-packet signal. The remote station receives and despread thethird BS-packet signal, i.e., the CO-packet signal.

[0021] This invention has the central office route the CO-packet signalto the second base station, since the second base station is the basestation to which the remote station last transmitted, as determined fromthe second base station source address and the remote station sourceaddress in the second packet signal previously received at the centraloffice.

[0022] The present invention refers to a first frequency f₁ and a secondfrequency f₂, which, for frequency division duplex (FDD), would bedifferent frequencies, preferably outside the correlation bandwidth ofeach other. The present invention also may be used with time divisionduplex (TDD), with the first frequency f₁ the same as the secondfrequency f₂. Both FDD technology and TDD technology are well know inthe art.

[0023] Additional objects and advantages of the invention are set forthin part in the description which follows, and in part are obvious fromthe description, or may be learned by practice of the invention. Theobjects and advantages of the invention also may be realized andattained by means of the instrumentalities and combinations particularlypointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The accompanying drawings, which are incorporated in andconstitute a part of the specification, illustrate preferred embodimentsof the invention, and together with the description serve to explain theprinciples of the invention.

[0025]FIG. 1 is a block diagram of a current cellular spread-spectrumsystem, showing all base stations communicating with a central office;

[0026]FIG. 2 is a block diagram of a distributed network,spread-spectrum system;

[0027]FIG. 3 illustrates a remote station communicating between two basestations; and

[0028]FIG. 4 illustrates a remote station communicating with a basestation, with channel sounding.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] Reference now is made in detail to the present preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings, wherein like reference numerals indicate likeelements throughout the several views.

[0030] The instant invention disclosed herein provides a novelimprovement and method to a direct-sequence, spread-spectrum,packetized, code-division-multiple-access (CDMA) system, and moreparticularly to a cellular structure or environment with each cellcontaining a base station communicating with a plurality of remotestations, using spreadi spectrum modulation. The present invention isfor packet data, with data being sent between a remote station and abase station as packet signals. A remote station might be a hand-heldunit or telephone, a connection to a computer or other modem, or otherdevice which may be stationery or in motion.

[0031] The base station is assumed to transmit to the plurality ofremote stations at a first frequency f₁, also known as a carrierfrequency of the base station transmitter. The plurality of remotestations is assumed to transmit to the base station at a secondfrequency f₂, also know as the carrier frequency of the remote stationtransmitters. For frequency division duplex operation, the secondfrequency f₂ is different the first frequency f₁, and typically outsidethe correlation bandwidth of the first frequency f₁. For time divisionduplex (TDD) operation, the second frequency f₂ is the same as the firstfrequency f₁. The present invention works with either a FDD CDMA systemor a TDD CDMA system.

[0032] A particular channel from the base station to a remote station isdefined or determined by a particular chip-sequence signal, as is wellknown in the art for direct-sequence (DS) code-division-multiple access(CDMA) systems. A particular channel from a particular remote station tothe base station is defined or determined by a particular chip-sequencesignal, as is well known in CDMA systems.

[0033] CDMA Network Architecture

[0034] The improvement to a method and system of the instant inventionprovides handoff for a spread-spectrum CDMA network. Thespread-spectrum, CDMA network may be a star network or a distributednetwork.

[0035] As illustratively shown in FIG. 1, a star network, as presentlyis used for cellular networks, is used to communicate data between acentral office 50 and a plurality of remote stations (RS) . A pluralityof base stations 20, 30, 40, communicate directly with the centraloffice 50. A first base station 20 communicates data between a firstplurality of remote stations 11, 22, 23, 24. A second base station 30communicates data between a second plurality of remote stations 31, 32,33, 34, 35, 36. A third base station 40 communicates data between athird plurality of remote stations 41, 42, 43, 44, 45.

[0036] The distributed network, as illustrated in FIG. 2, provides analternative architecture, for routing packet signals between a centraloffice, through a plurality of nodes, to a remote station. Each of thenodes in a distributed system includes a base station, plus additionalsystem components for the distributed system. A representativedistributive is incorporated herein by reference, and is disclosed inU.S. patent application Ser. No. 09/729,911, with filing date of Dec. 6,2000, entitled, DISTRIBUTED NETWORK, SPREAD-SPECTRUM SYSTEM, byinventors Donald L. Schilling and Joseph Gardnick, now U.S. Pat. No.------ , with issue date

[0037] As illustratively shown in FIG. 2, a distributed network,direct-sequence, spread-spectrum, packetized,codedivision-multiple-access (CDMA) system, by way of example, comprisesa plurality of remote stations and a plurality of nodes 110, 120, 130,140, 150, 160, 170 180, 190. The plurality of nodes 110, 120, 130, 140,150, 160, 170 180, 190 forms the distributed network. The distributednetwork plus the plurality of remote stations form the distributedsystem. The plurality of nodes 110, 120, 130, 140, 150, 160, 170 180,190 of FIG. 2, depicts, as an illustration, a first node 110, a secondnode, 120, a third node 130, a fourth node 140, a fifth node 150, asixth node 160, a seventh node 170, an eighth node 180 and a ninth node190.

[0038] In the plurality of nodes 110, 120, 130, 140, 150, 160, 170 180,190, one node, which happens to be labeled the second node 120, is a hubnode, which communicates to a central telephone office 50. In analternative embodiment, a set of the plurality of nodes (hubs)communicates to the central office 50. The set of the plurality of nodes(hubs), may include the entire plurality of nodes.

[0039] The plurality of nodes 110, 120, 130, 140, 150, 160, 170 180, 190covers a geographic area. Each node in the plurality of nodes 110, 120,130, 140, 150, 160, 170 180, 190 forms a micro-cell. A micro-celltypically has a radius much less than one mile.

[0040] In the plurality of nodes 110, 120, 130, 140, 150, 160, 170 180,190, the first node 110 communicates with the second node 120, thefourth node 140 and the fifth node 150. The second node 120 communicateswith the first node 110, the third node 130, the fourth node 140, thefifth node 150 and the sixth node 160. The third node communicates withthe second node 120, the fifth node 150 and the sixth node 160. Thefourth node communicates with the first node 110, the second node 120,the fifth node 150, the seventh node 170 and the eighth node 180. Thefifth node communicates with the first node 110, the second node 120,the third node 130, the fourth node 140, the sixth node 160, the seventhnode 170, the eighth node 180 and the ninth node 190. The sixth node 160communicates with the second node 120, the third node 130, the fifthnode 150, the eighth node 180 and the ninth node 190. The seventh node170 communicates with the fourth node 140, the fifth node 150 and theeighth node 180. The eighth node 180 communicates with the fourth node140, the fifth node 150, the sixth node 160, the seventh node 170 andthe ninth node 190. The ninth node communicates with the fifth node 150,the sixth node 160 and the eighth node 180.

[0041]FIG. 2 shows the first node 110 communicating with a firstplurality of remote stations 11, 112, 113, 114. The second node 120communicates with a second plurality of remote stations, with FIG. 2showing a first remote station 121 of the second plurality of remotestations. The third node 130 communicates with a third plurality ofremote stations 131, 132 and the fourth node 140, the fifth node 150 andthe sixth node 160 communicate with a fourth plurality of remotestations, a fifth plurality of remote stations, and a sixth plurality ofremote stations, respectively. FIG. 2 shows the fourth node 140communicating with a first remote station 141 of the fourth plurality ofremote stations, the fifth node 150 communicating with a first remotestation 151 of the fifth plurality of remote stations, and the sixthnode 160 communicating with a first remote station 161 of the sixthplurality of remote stations. The seventh node 170 and the eighth node180 are shown communicating with a seventh plurality of remote stations171, and an eighth plurality of remote stations 181, 182, respectively.The ninth node 190 communicates with a ninth plurality of remotestations, and FIG. 2 shows the ninth node 190 communicating with a firstremote station 191 of the ninth plurality of remote stations.

[0042] Handoff Between Base Stations

[0043] The improvement to the method and system of the instant inventionprovides handoff for a packetized, direct-sequence, spread-spectrum CDMAsystem, for the star network and distributed network, as represented inFIG. 1 or FIG. 2, respectively. Assume that a remote station (RS) 11, asshown, for example, in FIG. 3, is communicating with a first basestation (BS) 12 within a spread-spectrum, CDMA network.

[0044] For the spread-spectrum, CDMA network, the first base 12 stationtransmits, using radio waves, a first BS-packet signal to the remotestation 11. The first BS-packet signal is a spread-spectrum signal,since the first BS-packet signal is spread by a first BS-chip-sequencesignal at a first frequency f₁. Spread-spectrum technology for spreadinga packet signal, in general, is well known in the art. Examples includea spread-spectrum transmitter using a chip-sequence generator forgenerating a chip-sequence signal. The data in the packet signal isspread by a multiplying device or AND gate, by the chip-sequence signal.Equivalently, a replica of the chip-LAW sequence signal can be stored inmemory and outputted in response a proper symbol input, such as a bit.

[0045] The remote station 11, using a spread-spectrum receiver, receivesthe first BS-packet signal. A replica of the first BS-chip-sequencesignal, or equivalently a matched filter matched to the firstBS-chip-sequence signal, is used by the spread-spectrum receiver of theremote station for despreading BS-packet signals arriving from the firstbase station 20. The replica of the first BS-chip-sequence signal alsocould be used with a product detector and output filter for despreadingthe packet signal. The product detector and matched filter fordespreading a spread-spectrum signal are well known in the art.

[0046] The remote station 11 transmits, using radio waves, a firstRS-packet signal to the first base station 20. The first RS-packetsignal is spread by a first RS-chip-sequence signal at a secondfrequency f₂. Spread-spectrum transmitters and spread-spectrumtechnology for spreading the packet signals are well known in the art,as previously discussed for the first base station 20.

[0047] A replica of the first RS-chip-sequence signal, or equivalently amatched filter matched to the first RS-chip-sequence signal, is used bythe first base station 20 for despreading the first RS-packet signal.The first RS-packet signal includes a source address from the remotestation 11, and a destination address for the central office 50. Thedestination address routes packet signals to the recipient of the packetsignal. The source address from the remote station 11 later is used forrouting packet signals to the remote station 11. Source address anddestination address are well known in the art. The destination addressand the source address are put on the first RS-packet signal prior tospreading the first RS-packet signal.

[0048] Referring to FIG. 1, by way of example, the first base station 20receives the first RS-packet signal at the second frequency f₂. Thefirst base station 20 normally would despread the first RS-packetsignal, add or append its address to the packet signal, and then sendthe despread first RS-packet signal, by way of example, to a centraloffice 50 of FIG. 1. In FIG. 2, the eighth base station, located at node180, forwards a packet signal to a fifth base station, located at node150, and a second base station, located at node 120, en route to thecentral office 50. The RS-packet signals from the remote station couldbe forwarded through base stations to the central office by wiredchannel or wireless channel. Technology for despreading the RS-packetsignal is well known in the art, as discussed for the remote station 11.

[0049] In summary, The first RS-packet signal includes a source addressfrom the remote station, and the source address forwarded from the firstbase station. The despread first RS-packet signal includes the sourceaddress from the remote station, and the source address from the firstbase station.

[0050] In a preferred embodiment, a base station despreads a packetsignal, in general, and for this example, an RS-packet signal, andstores the RS-packet signal for transmission the central office. Theflow control mechanism, used by the nodes in FIG. 2, permits packetsignals to be forwarded in an efficient manner to one of several basestations en route to the central office 50. The use of flow control in adistributed network is well known to the ordinary skilled artisan in theart.

[0051] Referring to FIG. 3, while the remote station 11 communicateswith the first base station 20, the remote station 11 monitors, at thefirst frequency f₁, other base stations in geographic proximity to theremote station 11. Assume, as an illustrative example, that the remotestation 11 is required to handoff to another base station because thepower level is falling below a threshold or the capacity of the remotestation is falling below a threshold. The remote station 11 may monitorthe signal or power level and capacity availability of control andpacket transmission channels, or other channels, of the other basestations from nearby micro-cells. Assume that from a plurality of basestations monitored by the remote station, that the actual base stationfor handoff is defined herein as the second base station 134.

[0052] Prior to handoff, the second base station 134 transmits, usingradio waves, a second BS-packet signal, spread by a secondBS-chip-sequence signal at the first frequency f₁. The remote station 11monitors signal metrics of the first BS-packet signal from the firstbase station 20, and the same signal metrics of the other BS-packetsignals from the other base stations in the plurality of base stations.A signal metric typically would be received power or energy level, andcapacity level.

[0053] The remote station 11 determines when the signal power metric ofthe first BS-packet signal falls below a threshold and that the signalmetrics of one or more BS-packet signals are above the threshold. Theremote station 11 also determines from the other base stations, which ofthe other base stations has available capacity. Available capacity isability to store despread RS-packet signals and other packets signalspassing through the other base stations. The remote station 11 thenselects a base station, which is defined herein as second base station134, which meets the requirements of the signal metric and availablecapacity. The remote station 11 also determines which BS-chip-sequencesignals are available for use with the second base station 134. A set ofBS-chip-sequence signal and RS-chip-sequence signal are assigned andused by the remote station 11, when communicating with the second basestation 134.

[0054] Upon determining the foregoing threshold crossings, the remotestation 11 changes communications from the first base station 20 to thesecond base station 134, using a new set of second RS-chip-sequencesignal and second BS-chip-sequence signal. In response to determining tochange communications between base stations, the remote station 11transmits, using radio waves, a second RS-packet signal to the secondbase station 134. The second RS-packet signal is defined herein to be anRS-packet signal sent from the remote station to the second base station134.

[0055] After the second base station 134 receives and despreads thesecond RS-packet signal at the second frequency f₂, the second basestation 134 sends the despread second RS-packet signal to the centraloffice either as used in FIG. 1 or FIG. 2. The despread second RS-packetsignal includes a source address from the second base station 134, whichwas added to the second RS-packet signal by the second base station 134.

[0056] For return communications from the central office to the remotestation 11, the central office reads the source addresses from theRS-packet signal, of the remote station 11 and the forwarding basestation. For this example, the central office reads the source addressfor the second base station 134. From the source addresses, the centraloffice knows to route return packet signals, denoted herein as CO-packetsignal to the remote station 11 through the second base station 134.

[0057] The central office sends to the spread-spectrum, CDMA network, aCO-packet signal, including a destination address for the second basestation 134 and the remote station 11. The second base station 134receives the CO-packet signal of FIG. 1 or FIG. 2. The second basestation 134 transmits, using radio waves, the CO-packet signal, at thefirst frequency f₁, as a third BS-packet signal, respectively, eachspread by the second BS-chip-sequence signal. The third BS-packet signalis defined herein to be the CO-packet signal when the CO-packet signalis sent from the second base station 134 to the remote station 11. Theremote station 11 receives the CO-packet signal. The invention has thecentral office route the CO-packet signal to the second base station134, since the second base station 134 is the location from which theremote station 11 last transmitted, as determined from the second basestation 134 source address and the remote station 11 source address inthe second packet signal previously received at the central office.

[0058] Spread-Spectrum Channel Sounding

[0059] The present invention may be extended to include a soundingchannel. The sounding channel presents an additional signal metric fromwhich to make a determination when to handoff. In addition, the soundingchannel provides an accurate measure of the appropriate signal level forthe remote station, when transmitting to the base station. The soundingchannel is incorporated herein by reference, and is disclosed in U.S.patent application Ser. No. 09/231,015, with filing date of Jan. 14,1999, entitled, SPREAD-SPECTRUM CHANNEL SOUNDING, by inventor Donald L.Schilling, now U.S. patent number with issue date.

[0060] The addition of a sounding channel overcomes a major problem witha plurality of remote stations transmitting to a common base station.The plurality of remote stations may be located at different distances,and each remote station may have a different propagation path, to thebase station. Thus, even if all the remote stations transmitted with thesame power level, then the spread-spectrum signal from each remotestation may arrive at the base station with a different power level. Astrong power level from one remote station may cause sufficientinterference to block or inhibit reception of the spread-spectrum signalfrom a more distant remote station. This power problem is commonly knownas the “near-far” problem, or power control problem: How does thespread-spectrum system control the power transmitted from each remotestation, so that the power received at the base station from each remotestation is approximately the same? If the average power received at thebase station were the same for each remote station, then the capacity islimited by the number of remote stations transmitting to the basestation. If, however, a particular remote station were sufficientlyclose to the base station, and its transmitter power could blockreception of other remote stations, then capacity may be limitedseverely to only the remote station closest to the base station.

[0061] The sounding channel overcomes the power control problem bypermitting a remote station to have knowledge, a priori to transmitting,of a proper power level to initiate transmission. After the initialpower level is used, closed-loop power control, which is well-known inthe art, can be employed.

[0062] An additional or alternative benefit from the sounding is moreaccurate frequency control at a remote station. The carrier frequencytransmitted from a remote station may be shifted at the base station dueto Doppler shift in carrier frequency caused by motion. The soundingchannel initially corrects or compensates for Doppler shift in carrierfrequency caused by the effective motion of the remote station. Theremote station could be at a fixed location, and the Doppler shift incarrier frequency could be caused by time changes in the propagationpath, such as trees blowing in the wind. After initial communications, aCostas loop or other frequency controlling circuit may be employed tocontrol or compensate for frequency changes. Such devices or circuitsare well-known in the art.

[0063] As illustratively shown in FIG. 4, the sounding channel broadlyprovides an improvement to a spread-spectrum system which has a basestation and a plurality of remote stations operating in FDD mode. Thebase station has a BS-spread-spectrum transmitter and aBS-spread-spectrum receiver. The BS-spread-spectrum transmittertransmits, using radio waves, a plurality of BS-spread-spectrum signalsat a first frequency f₁. The BS-spread-spectrum receiver receives, at asecond frequency f₂, a plurality of RS-spread-spectrum signals from theplurality of remote stations. The plurality of BS-spread-spectrumsignals at the first frequency f₁ are outside the correlation bandwidthof the plurality of RS-spread-spectrum signals at the second frequencyf₂. Each of the plurality of remote stations has an RS-spread-spectrumtransmitter for transmitting an RS-spread-spectrum signal at the secondfrequency f₂.

[0064] The improvement includes a BS transmitter and aninterference-reduction subsystem, located at the base station receiver.The BS transmitter transmits, using radio waves, a BS-channel-soundingsignal at the second frequency. The BS-channel-sounding signal has abandwidth no more than twenty per cent of the spread-spectrum bandwidthof the plurality of RS-spread-spectrum signals, and in a preferredembodiment, the BS-channel-sounding signal has a bandwidth no more thanone percent of the spread-spectrum bandwidth of the plurality ofRS-spread-spectrum signals.

[0065] At each remote station, the improvement includes anRS-power-level circuit and an RS receiver which has an RS demodulator.The improvement at the remote station also may include afrequency-adjust circuit. The RS receiver receives theBS-channel-sounding signal at the second frequency. The RS demodulatortracks the BS-channel-sounding signal, and outputs an RS-receiversignal. Using the receiver power level of the RS-receiver signal, theRS-power-level circuit adjusts an RS-power level of theRS-spread-spectrum transmitter located at the remote station. If thefrequency-adjust circuit were employed, then the frequency-adjustcircuit, using the received RS-receiver signal as a reference,compensates to the first frequency the RS-spread-spectrum signal of theRS-spread-spectrum transmitter located at the remote station.

[0066] The interference-reduction subsystem is located at the basestation and at a front end to the BS-spread-spectrum receiver. Theinterference-reduction subsystem reduces, at the second frequency, theBS-channel-sounding signal from the plurality of RS-spread-spectrumsignals arriving at the base station.

[0067] In the exemplary arrangement shown in FIG. 4, the first basestation 20 is shown communicating, using radio waves, with a remotestation with frequency compensation. Since the BS-channel-soundingsignal is transmitted, as a radio wave, from the base station 20 at thesecond frequency f₂ to the remote station 11, and the remote station 11knows at what frequency the BS-channel-sounding signal is suppose to bereceived, then remote station 11 can determine the Doppler frequencyshift f_(D) and compensate its transmitter frequency by a similar amountso that the RS-spread-spectrum signal arrives at the base station 20with a carrier frequency at the correct second frequency f₂. Thus, theRS-spread-spectrum signal is detected at the base station at the secondfrequency f₂, without a Doppler shift in carrier frequency f_(D). Ifmotion of the remote station caused a positive shift in the Dopplerfrequency f_(D), then the correct compensation would be to subtract theDoppler shift in carrier frequency f_(D) and transmit at frequencyf₂−f_(D). The remote station 11 also can measure the power level of theBS-channel-sounding signal, and from this measurement, set its initialpower level for transmitting the RS-spread-spectrum signal at frequencyf₂.

[0068] For determining when to handoff to a second base station 134 ofFIG. 3, the remote station can monitor the signal metric of the soundingchannel. Handoff can be based on a signal metric of either the firstBS-packet signal, or the sounding channel, or a combination of the firstBS-packet signal and the sounding channel, as well as availablecapacity.

[0069] It will be apparent to those skilled in the art that variousmodifications can be made to the spread-spectrum channel soundingimprovement of the instant invention without departing from the scope orspirit of the invention, and it is intended that the present inventioncover modifications and variations of the spread-spectrum channelsounding improvement provided they come within the scope of the appendedclaims and their equivalents.

We claim:
 1. An improvement method to a spread-spectrum systemcomprising the steps of: transmitting, using radio waves, from a firstbase station (BS) a first BS-packet signal spread by a firstBS-chip-sequence signal at a first frequency; receiving at a remotestation (RS) the first BS-packet signal; transmitting, using radiowaves, from said first base station a first BS-channel-sounding signalat a second frequency; receiving at said remote station the firstBS-channel-sounding signal at the second frequency; tracking at saidremote station the first BS-channel-sounding signal, thereby outputtinga first RS-receiver signal; adjusting, in response to the firstRS-receiver signal, a first initial RS-power level of said remotestation; transmitting, using radio waves, from said remote station (RS),a first RS-packet signal spread by a first RS-chip-sequence signal at asecond frequency, at the initial RS-power level; receiving at said firstbase station the first RS-packet signal at the second frequency; sendingthe first RS-packet signal to a central office from the first basestation, with the first RS-packet signal including a source address fromthe first base station; transmitting, using radio waves, from a secondbase station a second BS-packet signal spread by a secondBS-chip-sequence signal at the first frequency; transmitting, usingradio waves, from said second base station a second BS-channel-soundingsignal at the second frequency, with the second BS-channel-soundingsignal having a bandwidth no more than twenty per cent of thespread-spectrum bandwidth of the first BS-packet signal spread by thefirst BS-chip-sequence signal at the first frequency; monitoring at saidremote station a signal metric of the first BS-packet signal and asignal metric of the first BS-channel-sounding signal, and a signalmetric of the second BS-packet signal and a signal metric of the secondBS-channel-sounding signal; determining at said remote station that thesignal metric of at least one of the first BS-packet signal and thefirst BS-channel-sounding signal fall below a threshold, and that thesignal metric of at least one of the second BS-packet signal and thesecond BS-channel-sounding signal are above the threshold, and that thesecond base station has available capacity, thereby determining tochange base stations; receiving at said remote station the secondBS-channel-sounding signal at the second frequency; tracking at saidremote station the second BS-channel-sounding signal, thereby outputtinga second RS-receiver signal; adjusting, in response to the secondRS-receiver signal, a second initial RS-power level of said remotestation; transmitting, using radio waves, from said remote station (RS),a second RS-packet signal spread by a second RS-chip-sequence signal atthe second frequency, at the second initial RS-power level;transmitting, using radio waves, in response to determining to changebase stations, from said remote station a second RS-packet signal spreadby a second RS-chip-sequence signal at the second frequency; receivingat said second base station the second RS-packet signal at the secondfrequency; sending the second RS-packet signal to a central office (CO)from the second base station, with the second RS-packet signal includinga source address from the second base station; sending from said centraloffice a CO-packet signal including a destination address for the secondbase station; receiving at said second base station the CO-packetsignal; transmitting, using radio waves, from said second base stationthe CO-packet signal the first frequency; and receiving at said remotestation the CO-packet signal.
 2. The improvement as set forth in claim1, wherein the step of transmitting from said first base station thefirst BS-channel-sounding signal includes the step of transmitting fromsaid first base station the first BS-channel sounding signal, with thefirst BS-channel-sounding signal having a bandwidth no more than twentyper cent of a spread-spectrum bandwidth of the first BS-packet signalspread by the first BS-chip-sequence signal at the first frequency. 3.The improvement as set forth in claim 1, wherein the step oftransmitting from said first base station the first BS-channel-soundingsignal includes the step of transmitting from said first base stationthe first BS-channel sounding signal, with the first BS-channel-soundingsignal having a bandwidth no more than ten per cent of a spread-spectrumbandwidth of the first BS-packet signal spread by the firstBS-chip-sequence signal at the first frequency.
 4. The improvement asset forth in claim 1, wherein the step of transmitting from said firstbase station the first BS-channel-sounding signal includes the step oftransmitting from said first base station the first BS-channel soundingsignal, with the first BS-channel-sounding signal having a bandwidth nomore than five per cent of a spread-spectrum bandwidth of the firstBS-packet signal spread by the first BS-chip-sequence signal at thefirst frequency.
 5. The improvement as set forth in claim 1, wherein thestep of transmitting from said first base station the firstBS-channel-sounding signal includes the step of transmitting from saidfirst base station the first BS-channel sounding signal, with the firstBS-channel-sounding signal having a bandwidth no more than one per centof a spread-spectrum bandwidth of the first BS-packet signal spread bythe first BS-chip-sequence signal at the first frequency.
 6. Animprovement method to a spread-spectrum system comprising the steps of:transmitting from a first base station (BS) a first BS-packet signalspread by a first BS-chip-sequence signal at a first frequency;receiving at a remote station (RS) the first BS-packet signal;transmitting from a second base station a second BS-packet signal spreadby a second BS-chip-sequence signal at the first frequency; monitoringat said remote station, a plurality of base stations, including a signalmetric of the first BS-packet signal and a signal metric of the secondBS-packet signal; determining at said remote station that the signalmetric of the first BS-packet signal falls below a threshold and thatthe signal metric of the second BS-packet signal is above the threshold,thereby deciding to change base stations; transmitting, in response todetermining to change base stations, from said remote station anRS-packet signal spread by an RS-chip-sequence signal at a secondfrequency; receiving at said second base station the RS-packet signal atthe second frequency; storing at the second base station the RS-packetsignal; forwarding the RS-packet signal to a central office (CO) fromthe second base station, with the RS-packet signal including a sourceaddress from the second base station; sending from said central office aCO-packet signal including a destination address for the second basestation; receiving at said second base station the CO-packet signal;transmitting from said second base station the CO-packet signal spreadby the second BS-chip-sequence signal the first frequency; and receivingat said remote station the CO-packet signal.
 7. The method as set forthin claim 6, with the step of transmitting from said remote station theRS-packet signal spread by the RS-chip-sequence signal at the secondfrequency includes the step of transmitting from said remote station theRS-packet signal at the second frequency, with the second frequencyequal to the first frequency.
 8. The method as set forth in claim 6,with the step of transmitting from said remote station the RS-packetsignal spread by the RS-chip-sequence signal at the second frequencyincludes the step of transmitting from said remote station the RS-packetsignal at the second frequency, with the second frequency outside acorrelation bandwidth of the first frequency.
 9. The method as set forthin claim 6, with the step of determining at said remote station thesignal metric further including the step of determining that the secondbase station has available capacity, in order to change base stations.10. The method as set forth in claim 7, with the step of determining atsaid remote station the signal metric further including the step ofdetermining that the second base station has available capacity, inorder to change base stations.
 11. The method as set forth in claim 8,with the step of determining at said remote station the signal metricfurther including the step of determining that the second base stationhas available capacity, in order to change base stations.
 12. Animprovement method to a spread-spectrum system comprising the steps of:transmitting from a first base station (BS) a first BS-packet signalspread by a first BS-chip-sequence signal at a first frequency;receiving at a remote station (RS) the first BS-packet signal;transmitting from said remote station (RS) a first RS-packet signalspread by a first RS-chip-sequence signal at a second frequency;receiving at said first base station the first RS-packet signal at thesecond frequency; sending the first RS-packet signal to a central officefrom the first base station, with the first RS-packet signal including asource address from the first base station; transmitting from a secondbase station a second BS-packet signal spread by a secondBS-chip-sequence signal at the first frequency; monitoring at saidremote station a signal metric of the first BS-packet signal and asignal metric of the second BS-packet signal; determining at said remotestation that the signal metric of the first BS-packet signal falls belowa threshold and the signal metric of the second BS-packet signal isabove the threshold, and that the second base station has availablecapacity, thereby changing base stations; transmitting, in response todetermining to change base stations, from said remote station a secondRS-packet signal spread by a second RS-chip-sequence signal at thesecond frequency; receiving at said second base station the secondRS-packet signal at the second frequency; sending the second RS-packetsignal to a central office (CO) from the second base station, with thesecond RS-packet signal including a source address from the second basestation; sending from said central office a CO-packet signal including adestination address for the second base station; receiving at saidsecond base station the CO-packet signal; transmitting from said secondbase station the CO-packet signal the first frequency; and receiving atsaid remote station the CO-packet signal.
 13. An improvement to aspread-spectrum system comprising: a first base station (BS) fortransmitting, using radio waves, a first BS-packet signal spread by afirst BS-chip-sequence signal at a first frequency, and fortransmitting, using radio waves, a first BS-channel-sounding signal at asecond frequency; a remote station (RS) for receiving the firstBS-packet signal, for receiving the first BS-channel-sounding signal atthe second frequency, and for tracking the first BS-channel-soundingsignal, thereby outputting a first RS-receiver signal, said remotestation for adjusting, responsive to the first RS-receiver signal, afirst initial RS-power level of said remote station, and fortransmitting, using radio waves, a first RS-packet signal spread by afirst RS-chip-sequence signal at a second frequency, at the initialRS-power level; said first base station for receiving the firstRS-packet signal at the second frequency, and for sending the firstRS-packet signal to a central office from the first base station, withthe first RS-packet signal including a source address from the firstbase station; a second base station for transmitting, using radio waves,a second BS-packet signal spread by a second BS-chip-sequence signal atthe first frequency, for transmitting, using radio waves, a secondBS-channel-sounding signal at the second frequency; said remote stationfor monitoring a signal metric of the first BS-packet signal and asignal metric of the first BS-channel-sounding signal, and a signalmetric of the second BS-packet signal and a signal metric of the secondBS-channel-sounding signal, said remote station for determining that thesignal metric of at least one of the first BS-packet signal and thefirst BS-channel-sounding signal fall below a threshold, and that thesignal metric of at least one of the second BS-packet signal and thesecond BS-channel-sounding signal are above the threshold, and that thesecond base station has available capacity, thereby determining tochange base stations; said remote station for receiving the secondBS-channel-sounding signal at the second frequency, and for tracking thesecond BS-channel-sounding signal, thereby outputting a secondRS-receiver signal, and for adjusting, in response to the secondRS-receiver signal, a second initial RS-power level of said remotestation; said remote station (RS) for transmitting, using radio waves, asecond RS-packet signal spread by a second RS-chip-sequence signal atthe second frequency, at the second initial RS-power level, and fortransmitting, using radio waves, in response to determining to changebase stations, a second RS-packet signal spread by a secondRS-chip-sequence signal at the second frequency; said second basestation for receiving the second RS-packet signal at the secondfrequency, and for sending the second RS-packet signal to a centraloffice (CO), with the second RS-packet signal including a source addressfrom the second base station; said central office for sending aCO-packet signal including a destination address for the second basestation; said second base station for receiving the CO-packet signal;said second base station for transmitting, using radio waves, theCO-packet signal the first frequency; and said remote station forreceiving the CO-packet signal.
 14. The improvement as set forth inclaim 13, wherein the said first base station includes means fortransmitting the first BS-channel sounding signal, with the firstBS-channel-sounding signal having a bandwidth no more than twenty percent of a spread-spectrum bandwidth of the first BS-packet signal spreadby the first BS-chip-sequence signal at the first frequency.
 15. Theimprovement as set forth in claim 13, wherein the said first basestation includes means for transmitting the first BS-channel soundingsignal, with the first BS-channel-sounding signal having a bandwidth nomore than ten per cent of a spread-spectrum bandwidth of the firstBS-packet signal spread by the first BS-chip-sequence signal at thefirst frequency.
 16. The improvement as set forth in claim 13, whereinthe said first base station includes means for transmitting the firstBS-channel sounding signal, with the first BS-channel-sounding signalhaving a bandwidth no more than five per cent of a spread-spectrumbandwidth of the first BS-packet signal spread by the firstBS-chip-sequence signal at the first frequency.
 17. The improvement asset forth in claim 13, wherein the said first base station includesmeans for transmitting the first BS-channel sounding signal, with thefirst BS-channel-sounding signal having a bandwidth no more than one percent of a spread-spectrum bandwidth of the first BS-packet signal spreadby the first BS-chip-sequence signal at the first frequency.
 18. Animprovement a spread-spectrum system comprising the steps of: a firstbase station (BS) for transmitting a first BS-packet signal spread by afirst BS-chip-sequence signal at a first frequency; a remote station(RS) for receiving the first BS-packet signal; a second base station fortransmitting a second BS-packet signal spread by a secondBS-chip-sequence signal at a first frequency; said remote station formonitoring a signal metric of the first BS-packet signal and a signalmetric of the second BS-packet signal, for determining that the signalmetric of the first BS-packet signal falls below a threshold and thatthe signal metric of the second BS-packet signal is above the threshold,thereby deciding to change base stations, and for transmitting, inresponse to determining to change base stations, an RS-packet signalspread by an RS-chip-sequence signal at a second frequency; said secondbase station for the RS-packet signal at the second frequency, and forsending the RS-packet signal to a central office (CO), with theRS-packet signal including a source address from the second basestation; said central office for sending a CO-packet signal including adestination address for the second base station; said second basestation for receiving the CO-packet signal, and for transmitting theCO-packet signal spread by the second BS-chip-sequence signal the firstfrequency; and said remote station for transmitting the CO-packetsignal.
 19. The improvement as set forth in claim 18, with said remotestation transmitting the RS-packet signal with the second frequencyequal to the first frequency.
 20. The improvement as set forth in claim18, with said remote station transmitting the RS-packet signal with thesecond frequency outside a correlation bandwidth of the first frequency.21. The improvement as set forth in claim 18, with said remote stationincluding means for determining that the second base station hasavailable capacity, in order to change base stations.
 22. Theimprovement as set forth in claim 18, with said remote station furtherincluding means for determining that the second base station hasavailable capacity, in order to change base stations.
 23. Theimprovement as set forth in claim 19, with said remote station furtherincluding means for determining that the second base station hasavailable capacity, in order to change base stations.
 24. An improvementto a spread-spectrum system comprising: first base-station (BS) meansfor transmitting, using radio waves, a first BS-packet signal spread bya first BS-chip-sequence signal at a first frequency, and fortransmitting, using radio waves, a first BS-channel-sounding signal at asecond frequency; remote-station (RS) means for receiving the firstBS-packet signal, for receiving the first BS-channel-sounding signal atthe second frequency, and for tracking the first BS-channel-soundingsignal, thereby outputting a first RS-receiver signal, said remotestation for adjusting, responsive to the first RS-receiver signal, afirst initial RS-power level of said remote station, and fortransmitting, using radio waves, a first RS-packet signal spread by afirst RS-chip-sequence signal at a second frequency, at the initialRS-power level; said first base-station means for receiving the firstRS-packet signal at the second frequency, and for sending the firstRS-packet signal to a central office from the first base station, withthe first RS-packet signal including a source address from the firstbase station; second base-station means for transmitting, using radiowaves, a second BS-packet signal spread by a second BS-chip-sequencesignal at the first frequency, for transmitting, using radio waves, asecond BS-channel-sounding signal at the second frequency; saidremote-station means for monitoring a signal metric of the firstBS-packet signal and a signal metric of the first BS-channel-soundingsignal, and a signal metric of the second BS-packet signal and a signalmetric of the second BS-channel-sounding signal, said remote station fordetermining that the signal metric of at least one of the firstBS-packet signal and the first BS-channel-sounding signal fall below athreshold, and that the signal metric of at least one of the secondBS-packet signal and the second BS-channel-sounding signal are above thethreshold, and that the second base station has available capacity,thereby determining to change base stations; said remote-station meansfor receiving the second BS-channel-sounding signal at the secondfrequency, and for tracking the second BS-channel-sounding signal,thereby outputting a second RS-receiver signal, and for adjusting, inresponse to the second RS-receiver signal, a second initial RS-powerlevel of said remote station; said remote-station (RS) means fortransmitting, using radio waves, a second RS-packet signal spread by asecond RS-chip-sequence signal at the second frequency, at the secondinitial RS-power level, and for transmitting, using radio waves, inresponse to determining to change base stations, a second RS-packetsignal spread by a second RS-chip-sequence signal at the secondfrequency; said second base-station means for receiving the secondRS-packet signal at the second frequency, and for sending the secondRS-packet signal to central-office (CO) means, with the second RS-packetsignal including a source address from the second base station; saidcentral-office means for sending a CO-packet signal including adestination address for the second base station; said secondbase-station means for receiving the CO-packet signal, and fortransmitting, using radio waves, the CO-packet signal the firstfrequency; and said remote-station means for receiving the CO-packetsignal.
 25. The improvement as set forth in claim 24, wherein the saidfirst base-station means includes means for transmitting the firstBS-channel sounding signal, with the first BS-channel-sounding signalhaving a bandwidth no more than ten per cent of a spread-spectrumbandwidth of the first BS-packet signal spread by the firstBS-chip-sequence signal at the first frequency.
 26. The improvement asset forth in claim 24, wherein the said first base-station meansincludes means for transmitting the first BS-channel sounding signal,with the first BS-channel-sounding signal having a bandwidth no morethan ten per cent of a spread-spectrum bandwidth of the first BS-packetsignal spread by the first BS-chip-sequence signal at the firstfrequency.
 27. The improvement as set forth in claim 24, wherein thesaid first base-station means includes means for transmitting the firstBS-channel sounding signal, with the first BS-channel-sounding signalhaving a bandwidth no more than five per cent of a spread-spectrumbandwidth of the first BS-packet signal spread by the firstBS-chip-sequence signal at the first frequency.
 28. The improvement asset forth in claim 24, wherein the said first base-station meansincludes means for transmitting the first BS-channel sounding signal,with the first BS-channel-sounding signal having a bandwidth no morethan one per cent of a spread-spectrum bandwidth of the first BS-packetsignal spread by the first BS-chip-sequence signal at the firstfrequency.
 29. An improvement a spread-spectrum system comprising thesteps of: first base-station (BS) means for transmitting a firstBS-packet signal spread by a first BS-chip-sequence signal at a firstfrequency; remote-station (RS) means for receiving the first BS-packetsignal; second base-station means for transmitting a second BS-packetsignal spread by a second BS-chip-sequence signal at a first frequency;said remote-station means for monitoring a signal metric of the firstBS-packet signal and a signal metric of the second BS-packet signal, fordetermining that the signal metric of the first BS-packet signal fallsbelow a threshold and that the signal metric of the second BS-packetsignal is above the threshold, thereby deciding to change base stations,and for transmitting, in response to determining to change basestations, an RS-packet signal spread by an RS-chip-sequence signal at asecond frequency; said second base-station means for the RS-packetsignal at the second frequency, and for sending the RS-packet signal tocentral-office (CO) means, with the RS-packet signal including a sourceaddress from the second base station; said central-office means forsending a CO-packet signal including a destination address for thesecond base station; said second base-station means for receiving theCO-packet signal, and for transmitting the CO-packet signal spread bythe second BS-chip-sequence signal the first frequency; and saidremote-station means for transmitting the CO-packet signal.
 30. Theimprovement as set forth in claim 29, with said remote-station meanstransmitting the RS-packet signal with the second frequency equal to thefirst frequency.
 31. The improvement as set forth in claim 29, with saidremote-station means transmitting the RS-packet signal with the secondfrequency outside a correlation bandwidth of the first frequency. 32.The improvement as set forth in claim 29, with said remote-station meansincluding means for determining that the second base station hasavailable capacity, in order to change base stations.
 33. Theimprovement as set forth in claim 29, with said remote-station meansfurther including means for determining that the second base station hasavailable capacity, in order to change base stations.
 34. Theimprovement as set forth in claim 30, with said remote-station meansfurther including means for determining that the second base station hasavailable capacity, in order to change base stations.